1. Field of the Invention
The present invention relates to an optical pickup device or optical head device and particularly to an optical system of an optical pickup device in an optical recording and reproducing apparatus.
2. Description of Related Art
For optical information recording mediums, various kinds of optical discs, called Laser Disc (LD), Compact Disc (CD), Digital Video Disc (DVD) and the like have been known. As an optical recording and reproducing apparatus for writing and reading information from an optical recording medium, there is an optical disc player. In such optical discs, normalization is made for each item such as the base thickness in different specifications. There is also a compatible disc player which can read information from those different optical discs.
In the compatible video disc player, an optical pickup device has an optical system for irradiating a light beam to the optical disc and reading the return light from the optical disc. A method for controlling a position of the optical pickup device at a high precision is always demanded in a tracking servo control of the optical pickup device. A knife edge method, a phase difference method, a push-pull method, or the like is used for the tracking servo control method.
For example, in the optical discs of CD and DVD, a numerical aperture NA, a thickness of substrate, an optimum reading light wavelength, and the like are designed on the basis of different specifications. To realize an optical pickup device of a CD/DVD compatible player, therefore, it is necessary to correct at least two differences regarding the numerical apertures NA and the thicknesses of the substrates.
An example of an optical pickup device with two focal points is realized using a hologram lens has been disclosed in Japanese Patent No. 2532818; JP-A-7-98431. This two focal point optical pickup device comprises a complex objective lens composed of a convex objective lens and a hologram lens. In the hologram lens, a plurality of diffraction grooves or ring-shaped concave and convex portions are concentrically formed on a transparent flat plate over the region corresponding to the numerical aperture for the CD. These diffraction grooves diffract an incident light beam so as to divide it into a zero-order, a first-order and higher-order diffraction light beams. Thus, the hologram lens functions as a concave lens for the first-order diffraction light beam, whereby each focal point is formed on corresponding one of recording surfaces of the optical discs. At this time, the light beam is allowed to transmit through the region where no diffraction groove is formed, without being diffracted, and this transmitted light is received by the objective lens together with the zero-order diffraction light. As a result, the transmitted light, the zero-order diffraction light, and the first-order diffraction light will have different numerical apertures, respectively. The first-order diffraction light diffracted by diffraction grooves is employed for reading a CD with a small numerical aperture. The transmitted light and the zero-order diffraction light of larger numerical numbers are employed for reading a DVD.
The structure of the above conventional the two focal point optical pickup device principally involves the formation of reading spots with a common single light source. This light source emitting a reading light of a wavelength as great as 650 nm, which is most suitable for the DVD, is normally used. However, a light most suitable for reading out a recordable/reproducible recording medium has a wavelength as great as 780 nm, which is called a CD-R (CD Recordable or an R-CD: Recordable CD) belonging to the CD group. Specifically, the light source having the wavelength as great as 650 nm is not suitable for reading out information recorded on the CD-R.
Therefore, in order to achieve a compatible player capable of performing a good recording/reproducing of information for the CD-R in addition to the DVD/CD, it is necessary to overcome the problem of at least three differences regarding the numerical apertures NA, substrate thicknesses, and wavelength 780 nm of reading light. To realize an optical pickup device of a compatible player of LD/CD/DVD/CD-R, it is necessary to construct an optical pickup device using a plurality of light sources of different wavelengths, each light source being suitable for corresponding one of the different discs, after resigning the construction using the single light source for a single wavelength.
When an optical system such as prism, lens, or the like is easily constructed by using a plurality of light sources, however, the whole of the optical pickup device is complicated and there is a tendency of an increase in size.
Since an emission wavelength of each semiconductor laser sensitively changes in accordance with the change of ambient temperature, it is necessary to perform a temperature management. If there is a plurality of semiconductor lasers, therefore, a countermeasure for heat radiation of them is important.